1. Field of the Invention
The present invention relates in general to multijunction photovoltaic devices and, more particularly, to improvements in the junctions between adjacent cells in a multijunction amorphous silicon photovoltaic device.
2. Description of the Related Art
Photovoltaic devices are used to convert solar radiation into electrical energy. This conversion is achieved as a result of what is known as the photovoltaic effect. When solar radiation strikes a photovoltaic cell and is absorbed by an active region of the cell, pairs of electrons and holes are generated. The electrons and holes are separated by an electric field built into the cell. In accordance with a known construction of solar cells using amorphous silicon, the built-in electric field is generated in a structure consisting of p-type, intrinsic and n-type layers (PIN) of hydrogenated amorphous silicon (a-Si:H). In solar cells having this construction, the electron-hole pairs are produced in the intrinsic layer of the cell when solar radiation of the appropriate wavelength is absorbed. The separation of the electrons and holes occurs under the influence of the built-in electric field, with the electrons flowing toward the region of n-type conductivity and the holes flowing toward the region of p-type conductivity. This flow of electrons and holes creates the photovoltage and photocurrent of the photovoltaic cell.
Some of the incident light is absorbed by the doped layers (the p-layer and the n-layer) but the carriers generated in these layers have an extremely short carrier lifetime and recombine before they can be collected. Hence, absorption in the doped layers does not contribute to the photocurrent of the photovoltaic cell and a minimization of absorption in doped layers enhances the short-circuit current of PIN photovoltaic cells. Absorption loss in the p-layer is a function of the bandgap of the p-layer. Thus, by adjusting the bandgap of the p-layer, the absorption loss in the p-layer can be minimized by including in the p-layer a bandgap widening material such as carbon, nitrogen, oxygen or fluorine. For example, the p-layer can be provided as hydrogenated amorphous silicon carbide (a-SiC:H) with p-type doping.
However, the addition of bandgap widening material to the p-layer increases its resistance. Therefore the amount of bandgap widening material that is added is usually limited by the amount of resistance considered tolerable in the device.
The n-type layer functions to form a rectifying junction with the intrinsic layer. In order to enhance this function, it is desirable to provide the n-layer with a high conductivity. However, it is also desirable to provide the n-layer with a wide optical bandgap since, as described above, carriers generated therein do not contribute to the photocurrent of the cell. Unfortunately, as is the case of the p-layer, the addition to the n-layer of any of the bandgap widening elements described above results in an increase in the resistance of the n-layer. Therefore, the n-layer is typically provided with a concentration of a bandgap widening element that is limited by the amount of resistance considered tolerable in the device.
It is desirable to increase the total number of photons of differing energy and wavelength which are absorbed in order to maximize the photocurrent output of a photovoltaic device. One technique for increasing photon absorption, and thereby increase device efficiency, is to provide a multijunction photovoltaic device with two or more photovoltaic cells arranged in a stacked configuration, i.e., one on top of the other. Such a multijunction photovoltaic device, also known in the art as a tandem junction solar cell, is disclosed in U.S. Pat. No. 4,272,641 issued to Hanak (the '641 patent), which is incorporated herein by reference. In particular, that patent teaches the construction of tandem junction amorphous silicon solar cells, wherein each cell has the above described PIN structure.
Such multijunction photovoltaic devices consist of a stack of two or more photovoltaic cells which are both electrically and optically in series. Typically in such devices, short wavelength light is absorbed in a first, top-most cell, and longer wavelength light is absorbed in second and, if present, subsequent cells. The first, second and subsequent photovoltaic cells of the multijunction device preferably respectively have successively narrower optical bandgaps in order to efficiently absorb solar radiation.
In order for such multifunction PIN photovoltaic devices to operate at maximum efficiency, current must flow unimpeded from each photovoltaic cell to the next adjacent cell in the stack of cells. However, the nature of the multijunction PIN photovoltaic device, i.e., p-i-n-p-i-n . . . , results in an n-p junction occurring at each interface between adjacent PIN cells and therefore in series electrically with those adjacent cells. Disadvantageously, each of these n-p junctions represents a diode having a polarity opposite to that of the photovoltage generated by each of the adjacent photovoltaic cells. The n-p junctions are non-linear elements that oppose the flow of photocurrent and thereby impose a substantial power loss on the device.
FIG. 1 illustrates a plot of current vs. voltage (IV) of a multijunction PIN photovoltaic device. In particular, curve 100 (broken line) represents the IV characteristic for such a photovoltaic device in which no steps have been taken to overcome the adverse effect of the n-p junctions at the interfaces between adjacent cells. As illustrated by curve 100, an inflection occurs in the region where the photocurrent of the device changes direction. Such an inflection represents an undesirable increase in the series resistance of the device due to the n-p junction. This aspect of the IV curve, characteristic of the n-p junction, limits the amount of photocurrent that can be conducted by the photovoltaic device, and therefore lowers the fill factor and power generation capability of the device. As used herein, the fill factor of a photovoltaic device is the ratio V.sub.mp I.sub.mp /I.sub.L V.sub.OC, where V.sub.mp and I.sub.mp are respectively the voltage and current at maximum power delivery of the device, and V.sub.OC and I.sub.L are respectively the maximum voltage and current achievable in the device.
A solution to the above described problem caused by the n-p junctions is to modify the structure of the multijunction device so that the junction occurring between each pair of adjacent cells performs like a tunnel junction. One known method for creating a tunnel junction between adjacent solar cells of a multijunction photovoltaic device constructed from crystalline semiconductor materials, such as silicon, is to heavily dope the respective n- and p-layers of the n-p junction formed by the adjacent cells. However, this method for creating a tunnel junction cannot readily be applied to the above described multijunction PIN devices because amorphous silicon is not easily doped to yield a highly conducting film. Such difficulty in achieving suitably high conductivity is particularly the case with wide bandgap alloys such as hydrogenated amorphous silicon carbide (a-SiC:H) and hydrogenated amorphous silicon nitride (a-SiN:H) which are preferred materials for constructing the p- and n-type layers of amorphous silicon PIN photovoltaic devices since, as described above, their use tends to maximize the optical transmissivity of each photovoltaic cell of the multijunction device. As a result, an attempt to highly dope the p- and n-layers of an amorphous silicon multijunction PIN device constructed with wide bandgap alloys does not achieve a desirable tunnel junction characteristic at the n-p junction between adjacent cells.
A method for creating a tunnel junction between adjacent solar cells of an amorphous silicon multijunction PIN device is disclosed in the above-incorporated U.S. Pat. No. 4,272,641. There, an additional tunnel junction layer is disposed between adjacent PIN cells, such layer being provided as a cermet incorporating a metal, or as a thin metal layer and a cermet, hereinafter the "metallic layer." While the metallic layer may function in conjunction with the adjacent cell layers to reduce the above described inflection in the IV curve 100 of the device (FIG. 1), the provision of the extra metallic layer substantially inhibits the manufacturing process. In order to manufacture an amorphous silicon multijunction photovoltaic device using such a metallic layer, a first photovoltaic cell is formed in a first material deposition system, for example a glow discharge chamber as described in the '641 patent. Next, the device must be removed from the glow discharge chamber and placed in a second material deposition system where the metallic layer is deposited. For example, the '641 patent describes deposition of the metallic layer by a sputtering process. Then the device must be returned to the first deposition system where a second photovoltaic cell is formed on the metallic layer. Of course, if the device includes more than two cells, the process of transferring between the two deposition systems must be continued. The additional deposition system and the time required to manufacture a multijunction device in accordance with such a process results in an overall increased cost of the device and reduction in production yield.
Further, while the metallic layer disclosed in the '641 patent is optically transmissive, it has a lower index of refraction than that of the adjacent n- and p-type a-Si:H layers of the cells it is disposed between. As a result of the different indexes of refraction, light is undesirably reflected at the interfaces between the metallic layer and the adjacent a-Si:H layers.